Question 1031687
You want to know what size in cm is
the drawing of the room.
[ cm/ ft ] x [ length in ft ] = [ cm on drawing ]
{{{ ( 3/1 )*18 = 54 }}} cm
and
{{{ ( 3/1 )*20 = 60 }}} cm
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{{{ 54*60 = 3240 }}} 
The size of the classroom on the drawing is 3240 cm2